Dynamics of the Coordination Complexes in a Solid-State Mg Electrolyte

Coordination
complexes of magnesium borohydride show promising
properties as solid electrolytes for magnesium ion batteries and warrant
a thorough microscopic description of factors governing their mobility
properties. Here, the dynamics of Mg­(BH4)2-diglyme0.5 on the atomic level are investigated by means of quasielastic
neutron scattering supported by density functional theory calculations
and IR and NMR spectroscopy. Employing deuterium labeling, we can
unambiguously separate all the hydrogen-containing electrolyte components,
which facilitate Mg2+ transport, and provide a detailed
analytical description of their motions on the picosecond time scale.
The planar diglyme chain coordinating the central Mg atom appears
to be flexible, while two dynamically different groups of [BH4]− anions undergo reorientations. The latter
has important implications for the thermal stability and conductivity
of Mg­(BH4)2-diglyme0.5 and demonstrates
that the presence of excess Mg­(BH4)2 units in
partially chelated Mg complexes may improve the overall performance
of related solid-state electrolytes.